Earth drilling rigs, of the kind used to drill water wells, and for mineral exploration, etc., typically comprise a vehicle-mounted tilting mast, a drill head (sometimes referred to as a “gearbox” since a gear transmission is its principal component) movable up and down the mast by a hydraulic hoist, and a hydraulic motor carried by the drill head for rotating a drill string. A pneumatic hammer is typically provided at the bottom of the drill string for repeatedly striking an anvil at the top of a drill bit. The bit typically has an array of carbide buttons for cutting rock. Hydraulic fluid and compressed air are provided by pumps and a compressor mounted on the vehicle and operated by an engine also mounted on the vehicle.
Drilling requires skill and experience for several reasons. Efficient drilling requires selection of an appropriate drilling speed, and maintenance of an appropriate downfeed or hold-back force on the drill string. The magnitude of the force must be adjusted each time a drill pipe is added to the drill string, and the direction of the force must be changed from downward to upward when the number of pipe sections making up the drill string is sufficient that the weight of the drill string itself can supply the necessary downward force.
Operator skill and experience are especially important because unexpected conditions, frequently encountered in drilling operations, require rapid operator response. Such conditions include, for example, underground formations that can cause a drill bit to become stuck, underground voids, and the like.
When a pneumatic hammer is used at the bottom of the drill string, skill and experience are also required to avoid “crowding” of the drill bit. That is, if the drill string is advanced against the bottom of a bore hole before the air pressure delivered to the hammer is sufficiently high, the hammer can fail to operate, and the downward force exerted on the bit can cause breakage of the carbide buttons.